THE Jonma~ OF BIOLOGICAL CHEM~~RY

ATP has two significant effects on the mitochondrial tricarboxylate transporting system. First, it alters the concentration gradients at equilibrium for the substrates of this transporter. ATP (2 IDM) caused the uptake of 10 nmol of citrate into the mitochondria coincident with the output of a similar amount of L-malate. This redistribution was dependent on ATP transport, the effect being inhibited by atractyloside and mimicked by the nonmetabolizable derivative adenylyl imidodiphosphate. A mechanism to account for these observations is proposed. Secondly, preincubation of mitochondria with ATP resulted in a Zto 3-fold increase in the K,,, of the mitochondrial citrate transporter. This effect of ATP was not produced by ADP and Pi, nor by N, N, N1, Nl-tetramethyl-p-phenylenediamine and ascorbate. It was prevented by the addition of rotenone and antimycin A. This effect of ATP was observed in the presence of oligomycin and could not be attributed to a change in the content of the known tricarboxylate carrier inhibitor, palmitoyl-CoA, nor to the ATP concentration. The origin of possible regulatory factor (or factors) is discussed. the other hand leads to conditions of reduced ATP levels both in intact liver and isolated mitochondria (11) Transport of citrate from mitochondria to cytosol in liver may be important from a control standpoint since it provides acetylCoA for fatty acid synthesis (12), it activates acetyl-CoA carboxylase (13), and inhibits phosphofructokinase (14). Thus, any effect of ATP on the transport of citrate might result in modulation either of the glycolytic or lipogenic pathways. In the course of evaluating the influence of ATP on the tricarboxylate carrier, two separate effects of ATP have been established. In this communication, we shall present data indicating that ATP influences the kinetics of the tricarboxylate carrier, and it also appears to alter the concentration gradients at equilibrium for the substrates of this transporter.